Which type of solar cell is best for low power indoor devices?

Low power devices such as sensors and wireless communication nodes, focused towards indoor applications, face serious challenges in terms of harvesting nearby natural sources of energy for power. Nowadays, these wireless systems use batteries as source of energy. These batteries need to be replaced in due time and this factor plays a major role in determining the life of the device. Often, the cost of replacing the battery outweighs the cost of the device itself. Also from an environmental perspective, reducing battery waste is laudable. In order to obtain an “infinite” lifetime of the system, the device should be able to harvest energy from renewable resources in the device’s environment. Photovoltaic (PV) energy is an efficient natural energy source for outdoor applications. However, for indoor applications, the efficiency of classical crystalline silicon PV cells is much lower. Typically, the light intensity under artificial lighting conditions found in offices and homes is less than 10 W/m² as compared to 100-1000 W/m² under outdoor conditions. Moreover, the spectrum is different from the outdoor solar spectrum. Although the crystalline Si cell is still dominating the PV market, second generation solar cells, i.e. thin film technologies, are rapidly entering the market. The different PV cells are rated by their power output under standard test conditions (AM1.5 global spectrum and light intensity of 1000 W/m²) but those conditions are not relevant for indoor applications. The question therefore arises: which type of solar cell is best for indoor devices? This paper contributes to answering that question by comparing the power output of different thin film solar cells (CdTe, CIGS, amorphous Si, GaAs and an organic cell with active layer P3HT:PCBM) with the classical crystalline silicon solar cell as reference. This comparison is made for typical artificial light sources, i.e. an LED lamp, a “warm” and a “cool” fluorescent tube and a common incandescent and halogen lamp, which are compared to the outdoor AM1.5 spectrum as reference. All light sources (including the outdoor spectrum) are scaled to an illumination of 500 lux to obtain a correct comparison. The best artificial light source for all cell types is the incandescent lamp which, for Si and CIGS, improves the performance of the cell with a factor of 3 compared with AM 1.5. The LED lamp is the worst light source for indoor PV with a decrease in performance of a quarter for amorphous silicon to two thirds for crystalline silicon cells. The best solar cells for indoor use depend heavily on the light source. For an incandescent lamp, crystalline silicon remains the best. However, for an LED lamp or “warm” fluorescent tube, amorphous silicon is significantly better. For “cold” fluorescent tubes as light sources, CdTe solar cells perform the best

The influence of absorbing donors and acceptors on the efficiency for a stacked and a monolithic organic tandem solar cell

In order for organic bulk heterojunction solar cells to compete with the traditional inorganic cells, power conversion efficiencies of more than 10 % are desirable. A way to improve the efficiency is to use a tandem configuration. In this article, we study the influence of the energy levels (HOMO and LUMO) of donor and acceptor on the efficiency for a stacked and a monolithic organic tandem cell. First, we consider the case where only the donor of each subcell is the absorber active material. Then, we consider the situation where both the donor and acceptor are good absorbers; the photons absorbed in donor and acceptor are contributing to the output power of the solar cell. For our calculations, we always take into account the organic nature of the photovoltaic cell by imposing a minimal LUMO-difference, necessary for exciton dissociation. Ideal material characteristics are obtained from these calculations. They give us an idea how the configuration of the energy levels of the active materials should ideally be for stacked and monolithic organic tandem cells, and this for 2 situations: (i) only the donors absorbs light (ii) both donors and acceptors absorb light. One result is that the requirements for an almost optimal configuration for the stacked tandem cell are quite broad, permitting that the values of the bandgaps for optimal cells are not that strict. This is not the case for the monolithic configuration; especially the value of the bandgap Eg1 of the first subcell is more critical than for a stacked cell. Another result is that when both materials absorb light, the highest maximum attainable efficiency reached is the same as in the case where only one material absorbs light, but higher efficiencies are reached for materials which have not optimal energy levels

Guidelines for the bandgap combinations and absorption windows for organic tandem and triple-junction solar cells

Organic solar cells have narrow absorption windows, compared to the absorption band of inorganic semiconductors. A possible way to capture a wider band of the solar spectrum-and thus increasing the power conversion efficiency-is using more solar cells with different bandgaps in a row, i.e., a multi-junction solar cell. We calculate the ideal material characteristics (bandgap combinations and absorption windows) for an organic tandem and triple-junction solar cell, as well as their acceptable range. In this way, we give guidelines to organic material designers

Image segmentation with adaptive region growing based on a polynomial surface model

A new method for segmenting intensity images into smooth surface segments is presented. The main idea is to divide the image into flat, planar, convex, concave, and saddle patches that coincide as well as possible with meaningful object features in the image. Therefore, we propose an adaptive region growing algorithm based on low-degree polynomial fitting. The algorithm uses a new adaptive thresholding technique with the L∞ fitting cost as a segmentation criterion. The polynomial degree and the fitting error are automatically adapted during the region growing process. The main contribution is that the algorithm detects outliers and edges, distinguishes between strong and smooth intensity transitions and finds surface segments that are bent in a certain way. As a result, the surface segments corresponding to meaningful object features and the contours separating the surface segments coincide with real-image object edges. Moreover, the curvature-based surface shape information facilitates many tasks in image analysis, such as object recognition performed on the polynomial representation. The polynomial representation provides good image approximation while preserving all the necessary details of the objects in the reconstructed images. The method outperforms existing techniques when segmenting images of objects with diffuse reflecting surfaces

People tracking by cooperative fusion of RADAR and camera sensors

Accurate 3D tracking of objects from monocular camera poses challenges due to the loss of depth during projection. Although ranging by RADAR has proven effective in highway environments, people tracking remains beyond the capability of single sensor systems. In this paper, we propose a cooperative RADAR-camera fusion method for people tracking on the ground plane. Using average person height, joint detection likelihood is calculated by back-projecting detections from the camera onto the RADAR Range-Azimuth data. Peaks in the joint likelihood, representing candidate targets, are fed into a Particle Filter tracker. Depending on the association outcome, particles are updated using the associated detections (Tracking by Detection), or by sampling the raw likelihood itself (Tracking Before Detection). Utilizing the raw likelihood data has the advantage that lost targets are continuously tracked even if the camera or RADAR signal is below the detection threshold. We show that in single target, uncluttered environments, the proposed method entirely outperforms camera-only tracking. Experiments in a real-world urban environment also confirm that the cooperative fusion tracker produces significantly better estimates, even in difficult and ambiguous situations

The potential of tandem photovoltaic solar cells for indoor applications

Several studies have already determined the power potential of different types of photovoltaic solar cells for indoor applications. However, a detailed study whether or not the use of tandem solar cell (i.e. using two solar cells with different bandgaps in a row) is beneficial for indoor use is lacking. This paper attempts to fill this lacuna by comparing the power output of different tandem solar cells with single-junctions as reference. The comparisons are done by efficiency simulations of the solar cells and the light spectra of typical artificial light sources, i.e. a typical fluorescent tube, a high pressure sodium and metal halide lamp, a typical LED lamp and a common incandescent lamp, which are compared to the outdoor AM 1.5 spectrum as reference. More specifically, we study the influence of the energy levels of the bandgaps, not only for the usual monolithic configuration, but also for a stacked set-up. By determining the relative efficiency gain compared to single-junctions, we prove the limited usefulness of tandem solar cells for indoor applications

Shapes-from-silhouettes based 3D reconstruction for athlete evaluation during exercising

Shape-from-silhouettes is a very powerful tool to create a 3D reconstruction of an object using a limited number of cameras which are all facing an overlapping area. Synchronously captured video frames add the possibility of 3D reconstruction on a frame-by-frame-basis making it possible to watch movements in 3D. This 3D model can be viewed from any direction and therefore adds a lot of information for both athletes and coaches